In recent years, wireless devices, such as wireless communication devices and wireless radar devices, employing spread-spectrum techniques or Ultra Wide Band (UWB) have been examined and utilized. Especially, with the increase of speed and efficiency of the wireless devices, wireless devices using high-frequency waves such as millimeter waves or quasi-millimeter waves have attracted attention. In such wireless devices using the wide-band frequencies, sidelobe occurs in wide frequencies due to frequency diffusion. Therefore, in a structure of such a wireless device, a filter such as a Band-Pass Filter (BPF) which passes only a specific frequency but blocks unnecessary frequencies is required.
In a wireless device for transmitting waves, such a filter is inserted between a transmission antenna and a power amplifier so that waves except frequencies regulated by the Radio Law are not transmitted from the transmission antenna. On the other hand, in a wireless device for receiving waves, such a filter is inserted between a receiving antenna and a Low Noise Amplifier (LNA) so that interference of unnecessary frequencies can be prevented and that the LNA at a next stage can efficiently amplify only waves of a desired frequency band. As explained above, in a structure of a wireless device, a filter and an antenna are connected with each other.
One example of a high frequency filter used in such a wireless device is a filter having a planar distributed constant circuit such as a microstripline (refer to Patent Reference 1 and Patent Reference 2, for example). Here, when the microstripline on a dielectric substrate is formed to have various shapes, coils and capacitors can be formed in a planar distributed constant circuit, thereby achieving the above filter.
In addition, a method is disclosed to form a filter or a feed line together with an antenna on the same substrate (refer to Patent Reference 3, for example).
An antenna radiation pattern and an antenna radiation gain of an antenna device used in a wireless device are crucial factors of deciding performance of the antenna device. In order to achieve a desired antenna radiation gain or radiation pattern, an antenna device is disclosed to have an array antenna structure in which a plurality of antenna elements are arranged.
FIG. 1 is a plan view showing a structure of such a conventional antenna device having an array antenna structure.
The antenna device shown in FIG. 1 includes a plurality of antenna elements 1001, a feed line 1002, and a filter 1040, which are formed on a surface of a dielectric substrate 1004.
The plurality of antenna elements 1001, each of which is a microstrip patch antenna element, form the array antenna structure.
The feed line 1002 forms a microstripline connecting the filter 1050 with the plurality of antenna elements 1001.
A feed source (power source) 1003, which is positioned at a boundary between the filter 1040 and the feed line 1002, feeds power to each of the antenna elements 1001 via the feed line 1002. The line structure in the antenna device shown in FIG. 1 is a parallel feeding structure. In more detail, each length of the feed line 1002 is generally the same between a first branch point 1007 to each antenna element 1001, and the power is fed to each antenna element 1001 in the same phase. Moreover, the antenna device shown in FIG. 1 uses a coplanar feeding scheme, forming the antenna elements 1001 and the feed line 1002 on a surface of the same substrate. Since the coplanar feeding scheme can be realized in the dielectric substrate 1004 having a monolayer structure, the coplanar feeding scheme is quite useful to realize a simple and inexpensive array antenna structure.
In the meanwhile, frequency characteristics of a filter are decided by the number of filter stages in the filter. Therefore, more filter stages can increase an attenuation amount except a transmission band, thereby improving frequency characteristics.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 9-238002    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2003-60404    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2002-271130